Abstract
The paper presents an analysis of amounts, characteristics and morphological impact of large woody debris (LWD) in the Tres Arroyos stream, draining an old-growth forested basin (9.1 km2) of the Chilean Southern Andes. Large woody debris has been surveyed along a 1.5 km long channel section with an average slope of 0.07 and a general step-pool/cascade morphology. Specific wood storage is very high (656-710 m3 ha-1), comparable to that recorded in old-growth forested basins in the Pacific Northwest. Half of the LWD elements were located on the active floodplain, and around two-thirds of LWD elements were found in accumulations. Different types of log jam were observed, some heavily altering channel morphology (log-steps and valley jams), while others just line the channel edges (bankfull bench jams). Log-steps represent approximately 22% of all steps, whereas the elevation loss due to LWD (log-steps and valley jams) results in 27% loss of the total stream potential energy. About 1600 m3 of sediment is stored in the main channel behind LWD structures, corresponding to approximately 150% of the annual sediment yield.